Cities have long recognized street sweeping as providing both aesthetic and health benefits to its citizens. More recently, removal of small debris from urban streets has been shown to improve water quality by reducing storm runoff pollutants.
Initially, individuals were employed to clean streets of waste, debris, trash, and other undesirable materials using brooms and shovels. Eventually, manpower was replaced by mechanized sweepers, initially horse-drawn, but later mounted on truck bodies (motorized street sweepers). Early mechanized street sweepers utilized rotating brooms to sweep material from the streets.
A motorized street sweeper is a motor vehicle designed to clear streets, pavements, parking lots, and other traffic areas of debris, including litter, waste, and dirt. Some motorized street sweepers include rotary brushes that sweep materials into a holding area. These devices work as rotary brooms that are moved across the surface to be cleaned. While the brushes are able to dislodge and sweep material into the holding area, this type of motorized street sweeper also lifts large amounts of dust into the air, causing a breathing hazard for the operator and anyone in the area being cleaned. To reduce the amount of dust, some motorized street sweepers use water to wet the surface before sweeping, however, this can allow debris to adhere to the surface and not be lifted into the holding area.
Another type of motorized street sweeper uses a vacuum to suck debris into the holding area of the vehicle. A vacuum head is placed near the ground and material is drawn into the vacuum head. Debris is removed from the air stream and the air stream exhausted into the atmosphere. Although the exhaust may be filtered, fine contaminates escape into the air exhausted from the street sweeper.
More recent motorized street sweepers recycle exhaust air. The vehicle includes a blower for delivering pressurized air to one side of a sweeper head while withdrawing air from the opposite side of the sweeper head. The sweeper head directs the pressurized air along the surface of the ground to be cleaned. The debris-laden air is directed back into the vehicle to a separating bin where the debris settles and is collected. After removal of the debris, the exhaust is recirculated back to the blower instead of being released to the atmosphere. The motorized street sweeper can also include spinning brushes or other devices that direct debris toward the sweeper head.
Removal of the debris depends on air flow from an inlet on one side of the top of sweeper head to an outlet on the other side of the top of the sweeper head. The blower creates a positive pressure of air that is forced through the inlet, across the sweeper head, and through the outlet to deliver the debris-laden air to the separating bin. The velocity and angle of the air entering the sweeper head is sufficient to impinge on the surface to be cleaned and remove debris near the inlet. However, the cross-sectional area of the sweeper head may be greater than the cross-sectional area of the inlet, which causes the velocity of the air to decrease. In some cases, a pressure chamber and a suction chamber are located within the sweeper head. The suction chamber increases in cross-sectional area as air flows from the inlet toward the outlet, which reduces air velocity. Impingement of the air stream on the surface to be cleaned further reduces the air velocity of the air stream near the surface, while air not contacting the surface has a higher velocity. Additionally, as the air sweeps across the sweeper head toward the outlet, the air rises away from the surface to be cleaned due to frictional heating with the surface to be cleaned and surfaces of the sweeper head, and also rises in order to take the shortest path toward the outlet. The slowing of the air flow and rising away from the surface to be cleaned causes debris being swept from the inlet side of the sweeper head toward the outlet side to drop out of the air stream, creating a “dead zone” under the outlet having insufficient air flow to lift the debris into the outlet. Furthermore, the velocity of the air may slow sufficiently that even debris located away from the outlet can fail to be swept away. Since the outlet can be located up to 10-12 inches away from the surface, debris under the outlet may not be picked up due to the lack of air velocity at the surface to be cleaned. Furthermore, air from the inlet side of the sweeper head may slow sufficiently prior to reaching the outlet that denser and heavier debris drops out of the air stream before reaching the outlet and is not picked up. These drawbacks can be particularly problematic with leaves and other flat debris, such as paper, which tend to “stick” to the surface to be cleaned.
Some motorized street sweepers use a “pressure bleed” system to help remove leaves and other flat debris. In a pressure bleed system, some of the air leaving the blower is exhausted to the atmosphere. In this manner, more air is being withdrawn from the blower than is delivered to the sweeper head, thus improving the removal of leaves and other flat debris. However, the exhaust from these systems can include micro-fine particulate matter.
The overall design of the sweeper head withdraws air upward and into the outlet. The low air velocity underneath the outlet, coupled with the outlet being located up to 10-12 inches away from the surface to be cleaned, can result in debris under the outlet not being removed. In particular, debris directly below the outlet experiences primarily upward air movement at low velocity. In an effort to remove debris in the dead zone, a motorized sweeper may travel slowly or may make a second pass over the surface. However, this is inefficient and costly. Furthermore, venting of exhaust into the atmosphere exposes operators and bystanders to fine particulate matter.